The present invention is generally directed to a method and means for eliminating cross-talk in liquid crystal display devices. The method is applicable to two-state display devices and also to gray level display devices. More particularly, the present invention is related to a display device in which means for preventing cross-talk between data lines and pixels is provided.
A proper understanding of the present invention can only be had by understanding the operation of a liquid crystal display device and the problems of parasitic capacitance inherent in the structure of these devices. In particular, a liquid crystal display device typically includes a pair of substrates fixed a specified distance apart. This distance is typically approximately 6 microns. A liquid crystal material is disposed between the substrates. The substrates are selected so that at least one of them is transparent. If back lighting is provided as a means for providing or enhancing the display and image, it is required that both substrates be substantially transparent. On one of these substrates there is disposed a transparent ground plane conductor typically comprising material such as indium tin oxide (ITO). The opposing substrate contains a rectangular array of individual electrode elements, called pixel electrodes. A semiconductor switch (preferably a thin film transistor) is associated with each of these pixel electrodes and is typically disposed on the substrate containing these electrodes. These transistor switches are usually based upon either amorphous silicon or polycrystalline silicon technology. At present, amorphous silicon technology is preferred because of its lower process temperature requirements. In effect, the aforementioned structure results in a rectangular array of capacitor-like circuit elements in which liquid crystal material acts as a dielectric. Application of voltage to a pixel electrode results in an electro-optical transformation of the liquid crystal material. This transformation is the basis for the display of text or graphical information seen on the device. It is noted that the invention herein is particularly applicable to the above-described display device in that each of the pixel electrodes is associated with its own semiconductor switch which may be turned on or off so that each individual pixel element may be controlled by signals supplied to its associated semiconductor switch. These semiconductor devices essentially act as electron valves for the deposition of charge on individual pixel electrodes.
Each transistor is provided with a scan line signal and a data line signal. In general, there are M data lines and N scan lines. Typically, the gate of each transistor switch is connected to a scan line and the source or drain of the transistor switch is connected to a data line.
In operation, a signal level is established on each of the M data lines. At this point, one of the N scan lines is activated so that the voltages appearing on the data lines is applied to the pixel electrodes through their respective semiconductor switch elements. A necessary consequence of the arrangement described is that each pixel electrode is surrounded on both sides by data lines. One of the data lines is the data line associated with the pixel electrode. However, the other data line is associated with an adjacent pixel electrode. This latter data line carries a different information signal. Also inherent in this structure are certain capacitive features. In particular, the pixel electrode and its opposing ground plane electrode portion form a capacitive structure. In addition, there are parasitic capacitances between each data line and its surrounding pixel electrode elements. Moreover, there is a parasitic capacitance which exists between the source and drain of the semiconductor switch element. The parasitic capacitances permit undesired signals to be applied to the pixel electrodes.
In a typical operational sequence, desired voltage levels are established on the data lines and a scan line is activated so as to apply these voltages to a single row of pixel electrodes. After a time sufficient for charging the LC capacitor, a different scan line is activated and a different set of data voltages is applied to a different pixel row. Typically, an adjacent pixel row is selected for writing video information. Thus, in a typical operation, one row of the display device can be written at one time, from the top to the bottom of the screen. In television applications, this top to bottom writing occurs in approximately 1/30th or 1/60th of a second. Thus, in this time period, a complete image is displayed on the screen. This image may include both text and graphical information.
As is well known in the electrical arts, capacitive effects are generally proportional to area and inversely proportional to distance. Thus, in high resolution liquid crystal display devices, the parasitic capacitance effects are particularly undesirable because of the requirement for small spacing between the lines. In typical applications contemplated herein, such as a television environment, the pixel electrodes are approximately 100 microns on a side and separated by a space of approximately 10 microns with an area of approximately 10.times.10 microns being set aside from each pixel for the placement of its associated semiconductor switch element. Thus it is found that in high resolution thin film transistor matrix addressed liquid crystal displays, the parasitic capacitance between the data lines and the pixel electrode is not insignificant when compared to the pixel capacitance. It is also noted that the parasitic capacitance between the data lines and the pixel electrode is increased by the presence of the parasitic source to drain capacitance in the switch element itself. In operation of such a display, the voltage on a pixel is set during its row address time. The semiconductor switch is then turned off and the voltage should remain fixed until the display is refreshed. However, any change in the voltage on an adjacent data line produces a change in the voltage on the pixel. In many drive schemes, the RMS voltage on a data line typically varies between 0 and 5 volts, depending on how many elements in the column are turned on. This results in an uncertainty or cross-talk in the voltage on the pixel. The maximum value for this voltage is 2 * [(C.sub.D +C.sub.SD)/C.sub.LC ] * 5 volts. Here C.sub.D is the parasitic capacitance arising from proximity of the data lines to the pixel electrode; C.sub.SD is the parasitic sourcedrain switch capacitance; and C.sub.LC is the capacitance associated with the liquid crystal cell structures itself. The factor of 2 results from the fact that there are two data lines adjacent to each pixel electrode. In a design in which there are approximately 100 pixels per inch, this results in a maximum voltage error of approximately 0.2 volts RMS. While this is not critical for on-off displays, it is very significant for gray scale displays where changes in the voltage of 0.05 volts RMS are visible.
One method for reducing cross-talk of the kind discussed above is the use of a storage capacitor in parallel with C.sub.L.sbsb.C. This reduces the maximum error voltage. However, this method is undesirable because it usually requires additional processing steps, because it can cause additional defects to be present and because it reduces the active area of the pixel elements.